1. Field Of The Invention
This invention relates to high pressure reciprocating plunger-type and piston-type pumps used in the petroleum industry, and more particularly, to such a pump having a discharge valve positioned substantially coaxially with respect to the pump plunger.
2. Description of the Prior Art
It is common practice in the petroleum industry to employ high pressure plunger-type pumps in a variety of field operations relating to oil and gas wells, such as cementing, acidizing and fracturing, among others. An example of such a high pressure pump is the Halliburton Services HT-400 horizontal triplex pump manufactured by Halliburton Services of Duncan, Okla. These pumps are frequently used in pumping two-phase slurries. Two-phase slurries are those in which solid particles (the "solid phase") are suspended in a liquid (the "liquid phase"). A problem with pumping such two-phase slurries is that the solid phase particles can separate out of the carrier liquid and can collect in valves, elbows, and in the fluid ends of the high pressure pumps in the system.
In the pumps, these particles tend to become packed ahead of the pump plunger or piston. This can result in sudden overpressuring of the fluid in the pump with resulting damage to one or more of the plunger, connecting rod, crankshaft, fluid end, valves or other parts of the pump drive train.
One solution to the overpressuring problem is disclosed in U. S. Pat. No. 4,508,133 to Hamid, assigned to the assignee of the present invention. This invention comprises a protective cover assembly including a substantially circular cover having a shear disc surrounded by an annular outer portion, mounted in a cylinder in the fluid end of the plunger-type high pressure pump. An arcuate boundary of reduced wall thickness is between the shear disc and the outer portion of the cover. The cover is held in place by a retainer assembly which is secured to the fluid end, which retainer assembly includes a plug at the outer end of the retainer. When a predetermined force is generated by the plunger and the cylinder, such as in an overpressure situation, the shear disc of the cover shears and is propelled outwardly against the plug. In turn, this forces the impact disc against an edge of a circular recess in the outer end of the retainer. The impact disc, in shearing against the recess edge, safely dissipates the kinetic energy of the shear disc, while the pressure in the cylinder vents to the atmosphere, avoiding damage to the fluid end of the pump, the plunger, connecting rod, crankshaft, etc.
U. S. Pat. No. 4,520,837 to Cole et al., also assigned to the assignee of the present invention, discloses a protective cover with a shear disc essentially the same as in Hamid, but also includes a more simple, one-piece cover retainer inserted behind the protective cover.
One problem with the apparatus of Hamid and Cole et al. is that the shear disc is subjected to cyclic loading. The cyclic stress causes fatigue and premature failure of the disc around the thin arcuate wall may occur even at low pump pressures. Another problem is that the thin area around the arcuate portion does not leave much thickness for corrosion allowance, and thus may fail prematurely when corrosion is present. A further problem with the previous apparatus is that the shear disc is expensive to fabricate, and machining may leave machine marks which act as stress risers and compound the fatigue problem already mentioned.
An apparatus which solves the fatigue problem is disclosed in U. S. Pat. No. 4,771,801 to Crump et al., also assigned to the assignee of the present invention. In this invention, a cover with a convex or domed center portion is used and is adapted for buckling away from the pump plunger when the pressure in the pump exceeds a predetermined level. The convex portion buckles when excessive force is transmitted from the plunger through any packed solid particles, allowing the packed particles to be pushed through vent passages.
Even though the apparatus of Crump et al. greatly reduces the risk of fatigue failure over Hamid and Cole et al., all three have two additional problems. First, once failure has occurred, the pump must be taken out of service immediately upon rupture of the protective cover. Secondly, a large quantity of the pump fluid can be spilled out of the pump when the cover ruptures. This limits the suitability of the apparatus for pumping some hazardous fluids. Also, the use of such shear discs obviously adds somewhat to the operating costs of the pump in which they are employed.
The pump fluid end of the present invention solves the problems of the prior art by providing a pump in which a discharge valve is placed coaxially with the pump plunger or piston so that, if solid particles become packed ahead of the plunger, the force is transmitted through the particles to the discharge valve, pushing the valve open. This allows the packed particles to be pushed through the valve opening into the discharge passage. During this process, the pump may remain in use, and all fluids remain contained within the pump and its associated plumbing. The pressure containing envelope of the pump contains no designed in "weak links" which present the possibility of failing and spilling fluid, as is possible in the prior art.